My right leg is red and swollen from foot to just below knee for about a month?

DVT: The list of things which can cause redness and swelling in one leg is quite long and difficult to pinpoint without further information. The most worrisome cause would be a thrombosis (blood clot) involving a deep vein of the lower extremity also referred to as dvt. Urgent consultation with a physician is recommended as the complication of DVT can be dire. Infection is less likely at one month.